The present invention relates to metal lines in semiconductor devices and to methods for forming the same, and more particularly, to a metal line having a multi-layered diffusion barrier that has a decreased resistance as well has a desirable characteristic in preventing constituents in the metal line, such as copper, from diffusing beyond the bounds of the diffusion barrier.
Generally, metal lines are formed in a semiconductor device to electrically connect together elements or lines with each other. Contact plugs are formed to electrically connect lower metal lines and upper metal lines to each other. As the high integration of a semiconductor device continues to proceed, the aspect ratios of a contact hole in which a contact plug is to be formed gradually increases. As a result, the process for forming metal lines and contact plugs become more difficult because these diminutive metal lines and contact plugs must also maintain their physical and performance characteristics such as assuring that unwanted impurities do not diffuse past their corresponding barriers.
Aluminum and tungsten have been mainly used as conductive materials in the metal lines of a semiconductor device because they exhibit relatively good electrical conductivity properties and because they do not tend towards adversely affecting the performance of the resultant semiconductor device due to unwanted diffusion away from these metal lines. Recently, research has been made in the hopes of using copper as a next-generation material for a metal line because copper exhibits superior electrical conductivity and low resistance as compared to aluminum and tungsten. Copper (Cu) can therefore solve or at least aid in minimizing the problems associated with RC (resistance-capacitance) signal delay in the semiconductor device having a high level of integration and high operating speed.
It is known that copper diffuses very fast through semiconductor substrates and through insulation layers. Further the diffused copper is known to act as a deep-level impurity in the semiconductor substrate made of silicon and can induce leakage currents. Therefore, it is necessary to form a diffusion barrier at an interface between a copper layer acting as a metal line and the surrounding insulation layer.
Hereinbelow, a conventional method for forming a metal line of a semiconductor device will be briefly described.
After forming an insulation layer on a semiconductor substrate, a metal line forming region is defined by etching the insulation layer. Then, a diffusion barrier is formed on the insulation layer and on the surface of the metal line forming region, and a copper seed layer is subsequently formed on the diffusion barrier. Next, after forming a copper layer on the copper seed layer, by CMPing (chemically and mechanically polishing) the copper layer, a metal line comprising the copper layer is formed.
However, in the conventional art described above, as the size of cells decreases in conformity with the trend toward higher integration of these semiconductor devices, it is becoming more and more difficult to properly prevent the diffusion of a copper constituent by means of the diffusion barrier formed via the conventional arts.
Also, in the conventional art described above, it is difficult or even impossible to increase the thickness of the diffusion barrier so as to improve the characteristics of the diffusion barrier because of the design constraints of these diminutive structures. Due to this fact, the performance characteristics of the diffusion barrier are likely to deteriorate, and the resistance of the metal lines are likely to increase. As a result, in the conventional art described above, the characteristics and the reliability of a semiconductor device, including the characteristics of a metal line, are likely to deteriorate.